On another section here, we had a discussion about heat loss, in general, and comments were made about whether or not you lost much heat through a window via radiation. As I have often mentioned here, the GBA forum has some great blogs, like this one, the third one on windows. It is worth reading if you are interested in windows, replacement or new. Windows have become incredibly complex in the last 10 yrs or so.... as has a lot of building science. "And the beat goes on....."